Environmentally driven regulatory pressure concerning motor gasoline sulfur levels is expected to result in the widespread production of less than 50 wppm sulfur mogas by the year 2004. Levels below 10 wppm are being considered for later years in some regions of the world, and this will require deep desulfurization of naphthas in order to conform to emission restrictions that are becoming more stringent. The majority, i.e. 90% or more, of sulfur contaminants present in motor gasolines typically come from fluidized catalytically cracked (FCC) naphtha streams. However, FCC naphthas streams are also rich in olefins, which boost octane, a desirable quality in motor gasolines.
Thus, many processes have been developed to produce low sulfur products from olefinic naphtha boiling range streams while attempting to minimize olefin loss, such as, for example, hydrodesulfurization processes. However, these processes also typically hydrogenate feed olefins to some degree, thus reducing the octane number of the product. Therefore, processes have been developed that recover octane lost during desulfurization. Non-limiting examples of these processes can be found in U.S. Pat. Nos. 5,298,150; 5,320,742; 5,326,462; 5,318,690; 5,360,532; 5,500,108; 5,510,016; and 5,554,274, which are all incorporated herein by reference. In these processes, in order to obtain desirable hydrodesulfurization with a reduced octane loss, it is necessary to operate in two steps. The first step is a hydrodesulfurization step, and a second step recovers octane lost during hydrodesulfurization.
Processes other than those above have also been developed that seek to minimize octane lost during hydrodesulfurization. For example, selective hydrodesulfurization is used to remove organically bound sulfur while minimizing hydrogenation of olefins and octane reduction by various techniques, such as the use of selective catalysts and/or process conditions. One selective hydrodesulfurization process, referred to as SCANfining, has been developed by ExxonMobil Research & Engineering Company in which olefinic naphthas are selectively desulfurized with little loss in octane. U.S. Pat. Nos. 5,985,136; 6,013,598; and 6,126,814, all of which are incorporated by reference herein, disclose various aspects of SCANfining.
However, nitrogen-containing compounds present in refinery feedstreams are known to have a negative impact on the reaction rate of hydrodesulfurization processes. Using current industry technology nitrogen compounds are typically removed during hydroprocessing first by hydrogenation followed by hydrodenitrogenation. Thus, hydrodesulfurization processes that use catalysts having a high hydrogenation activity have been proposed to overcome the negative effects nitrogen compounds have on the hydrodesulfurization processes. However, the use of catalysts with high hydrogenation activity is typically not consistent with the need to preserve olefins during the hydrodesulfurization of olefinic naphthas.
Thus, there still exists a need in the art for an effective process to reduce the sulfur content in olefinic naphtha hydrocarbon streams, which contain nitrogen-containing compounds.